Lecanemab and donanemab are lab-made antibodies, and their job is simple to describe. They latch onto amyloid, the protein that clumps in the brains of people with Alzheimer’s, and flag it for the immune system to clear away. Sold as Leqembi and Kisunla, they are the first drugs that go after the disease itself rather than easing symptoms alone.
Two drugs, two targets
They do not grab the same thing. AARP, citing specialists it interviewed, reported that Leqembi binds to small amyloid fragments called protofibrils, while Kisunla binds to the plaques themselves. Dr. Andrew Budson of VA Boston and Boston University was frank about what that means in practice: “Whether these different binding targets are critical is not clear.”
The schedules differ too. Leqembi is given every two weeks for 18 months and then every two to four weeks, and Budson said nobody has clear guidance on when, if ever, to stop. Kisunla goes in once every four weeks, and a PET scan showing minimal amyloid is the signal to stop. Budson noted that about 70 percent of people on donanemab reached that stopping point after 18 months. In August 2025 the Food and Drug Administration also approved a once-a-week under-the-skin pen that patients can use at home to continue Leqembi after the first 18 months.
What the trials found
Alzheimer’s Research UK described Lilly’s TRAILBLAZER ALZ-2 trial, which enrolled more than 1,700 people with early Alzheimer’s. Among those with low to medium levels of tau, donanemab held back the slide in memory and thinking by 35% relative to placebo. Over a year, 47% of people on the drug saw no significant worsening, against 29% for placebo patients. In people with high tau, the drug had no significant effect, which is why doctors say it works best at the earliest stage.
Dr. Paul Schulz, a neurologist at UTHealth Houston who studied Kisunla in trials, called both drugs “transformational.” Others are more guarded. Dr. Mia Yang of Wake Forest School of Medicine pointed out that the “benefit is modest in cognition; both groups worsen over time.”
Risks and cost
The main safety concern is ARIA, short for amyloid-related imaging abnormalities, which means swelling or bleeding in the brain. It can be mild, moderate or severe, and when severe it can be life-threatening, and it is why patients need repeated MRI scans. People on Kisunla had more ARIA than people on Leqembi, though the two drugs were not tested against each other. A modified Kisunla dosing schedule brought the swelling risk down to 16 percent, close to the 12.6 percent seen with Leqembi, Budson said. Deaths in the trials were most often among patients who were also taking blood thinners.
List prices run higher than $25,000 a year. In Britain, the regulator licensed donanemab on 23 October 2024, but it will not be offered on the NHS because of its cost against the benefit.
If you or a relative has been diagnosed, talk to your doctor about whether these treatments are appropriate, and which scans and monitoring they would involve.
Sources
- Leqembi and Kisunla: What to Know About the Alzheimer's Drugs – AARP
- New Alzheimer's drug, donanemab – what is it and how does it work? – Alzheimer's Research UK
